Mobile communication systems require broad bandwidth and high linearity. Communications signals, used in such mobile communication systems, exhibit a high peak-to-average power ratio. Typical RF power amplifiers, that are used to amplify the communication signals, are operated at a large back-off power to satisfy the high peak-to-average power ratio and high linearity. Back-off corresponds to the difference between the increase in power of the output signal with the increase in power of the input signal. Typical RF amplifiers are described in conjunction with FIG. 1, FIG. 2, and FIG. 3.
FIG. 1 is a schematic diagram of a conventional multistage Class-AB power amplifier 100. Multistage Class-AB power amplifier 100 includes an input port 102, a multistage amplifier 104, and an output port 106. Multistage amplifier 104 includes an input amplifier 108, a driver amplifier 110, and an output amplifier 112. In an embodiment, input amplifier 108 is Class-A biased, driver amplifier 110 and output amplifier 112 are Class-AB biased.
Input port 102 is connected to input amplifier 108. Input amplifier 108, driver amplifier 110 and output amplifier 112 are connected in a cascade configuration. Output amplifier 112 is connected to output port 106.
Multistage power amplifier 100 offers limited output power and limited power added efficiency. In order to improve the output power, two multistage power amplifiers 100 are connected in parallel to form a balanced amplifier configuration as described in conjunction with FIG. 2.
FIG. 2 is a schematic diagram of a conventional balanced amplifier 200. Balanced amplifier 200 includes a first power splitter 202, a first multistage amplifier 204, a second multistage amplifier 206, and a second power splitter 208. First multistage amplifier 204 includes a first input amplifier 210a, a first driver amplifier 212a and a first output amplifier 214a. Second multistage amplifier 208 includes a second input amplifier 210b, a second driver amplifier 212b and a second output amplifier 214b. 
The amplifier stages in first multistage amplifier 204 and second multistage amplifier 206 are biased in a similar manner as the amplifier stages in multistage amplifier 104 (refer to FIG. 1).
First power splitter 202 is connected to first input amplifier 210a and second input amplifier 210b. In first multistage amplifier 204, first input amplifier 210a, first driver amplifier 212a and first output amplifier 214a are connected in cascade configuration. Similarly, in second multistage amplifier 206, second input amplifier 210b, second driver amplifier 212b, and second output amplifier 214b are connected in cascade configuration. First output amplifier 214a and second output amplifier 214b are connected to second power splitter 208.
An input signal, applied at first signal splitter 202, is split in a first signal and a second signal. The first signal and second signal are 90 degrees out of phase. The first and second signals are then amplified by first multistage amplifier 204 and second multistage amplifier 206 respectively. The amplified first and second signals are combined by second signal splitter 208 to generate an output signal.
As the signal is split (i.e., into the first signal and second signal) and amplified individually, the output power is improved as compared to multistage Class-AB power amplifier 100. In an embodiment, balanced amplifier 200 produces approximately twice the output power as produced by multistage Class-AB power amplifier 100. In order to achieve further improvement in the usable output power and to also improve the power added efficiency, balanced amplifier 200 is modified to form a Doherty amplifier.
FIG. 3 is a schematic diagram of a conventional Doherty amplifier 300. Doherty amplifier 300 includes a main amplifier 302, a peaking amplifier 304, and a Doherty combiner 308. Main amplifier 302 is Class B or Class AB biased. Peaking amplifier 304 is Class C biased. Further, it is known that Doherty amplifier 300 may also include more than one peaking amplifiers.
Doherty amplifier 300 achieves high efficiency at back-off through main amplifier 302 which operates into the high power added efficiency saturation region. Further, due to class-C biasing in peaking amplifier 304, peaking amplifier 304 supplies the signal peaks so that overall linearity can be restored. Additionally, the Doherty amplifier 300 achieves load modulation by using the principle of “load pulling” using two devices (i.e., main amplifier and peaking amplifier).
Main amplifier 302 operates when main amplifier 302 receives a low power input signal. As the power of the input signal increases, the Class-C amplifier (i.e., peaking amplifier 304) turns ON abruptly. Such abrupt turning ON of the Class-C amplifier leads to strong AM-AM distortion and AM-PM distortion. AM-AM distortion leads to undesired amplitude deviations while amplifying the peaks of the communication signal. Similarly, AM-PM leads to undesired phase deviations while amplifying the peaks of the communication signal. As most of the analog communication signals carry digital symbols, AM-AM distortion and AM-PM distortion may impede the ability to recognize the digital symbols leading to a distortion known as Error Vector Magnitude (EVM).
A person having ordinary skill in the art will understand that AM-AM distortions and AM-PM distortions in the output signal can be introduced due to various other factors such as the non-linear characteristics of amplifiers in the Doherty amplifier, and sudden gain compression and expansion in the amplifiers.
A person having ordinary skill in the art will also understand that the linearity degrades from Class-A to B and then to C while, in general, the current consumption decreases and power added efficiency (PAE) increases. Thus Class-A amplifier exhibits better linearity but at the cost of worse power added efficiency. Similarly, Class-C amplifier exhibits worse linearity but with the advantage of much improved power added efficiency. Such non-linear characteristics of Class-C amplifier introduce inter-modulation distortions among one or more input signals applied to the Doherty amplifier.
An addition to the distortions, most of the Doherty amplifiers are bulky as well as costly. Thus, such Doherty amplifiers may be unsuitable for various applications such as, but not limited to, active antenna systems, Femto cells, and mobile devices.
Thus, there is a need for an amplifier configuration that exhibits high power added efficiency and gain.